1. Field
The present invention relates generally to communications, and more specifically to transmission format detection in a variable-format transmission scheme.
2. Background
Traditionally, communication systems provided voice service but little else. Voice data was broken into small pieces and coded according to a single format for transmission. Today's communication systems offer a variety of services which vary dramatically in their requirements, such as quality of service requirements, delay requirements, error or rate requirements, and data rate requirements. This places a significant burden on the communication system to provide many different transmission formats to accommodate these services in an efficient manner. Further, a single communication can include two or more of these transmission formats simultaneously. For example, a single call can include audio, video, and data (such as text characters or graphical information). These types of data have different tolerances to delay and varying requirements in terms of quality of service. So each may be encoded differently with different size and different protection schemes.
Today, many second-generation and third-generation mobile communication systems employ multiple transmission format schemes to meet the needs of varied services. These schemes are used to transmit information in the form of digital data, where the data is alternatively encoded according to two or more transmission formats and sent over a communications link. Transmission formats can vary according to type of data (e.g., video, audio, data), modulation, transmission rate (often referred to as variable rate transmission schemes), error protection schemes, or transmission payload sizes. For example, the mobile systems specified in the Telecommunications Industry Association/Electronics Industry Association-95 (TIA/EIA-95) and the 3rd Generation Partnership Project—Universal Mobile Telecommunications System (3GPP-UMTS) standards employ multiple transmission format schemes. These schemes will be referred to herein collectively as variable-format transmission schemes.
Receivers used in a variable-format transmission scheme detect the actual transmission format used by the transmitter. Format detection can be explicit or implicit. In implicit detection schemes, the transmitter does not provide any information to the receiver identifying the transmission format used by the transmitter during the encoding process. Implicit schemes commonly employ a trial and error approach wherein the receiver tries to decode the data according to permissible formats until the data is correctly decoded. Successful decoding can be verified, for example, by appending a cyclic (or cyclical) redundancy code (CRC) to the data block. If the CRC “checks” it is very likely that the data is correct. If the CRC does not check, the data block either contains one or more bit errors or has been decoded with an incorrect transmission format assumption.
In explicit detection schemes, the transmitter provides the receiver with information, referred to herein as side information, that identifies the particular transmission format used in the encoding process. The receiver decodes the received information assuming that the transmission format is the one indicated by the transmitter. As with implicit detection, the receiver can verify that the data is correctly decoded if a CRC is appended by the transmitter.
Transmissions are broken into transmission intervals referred to herein as frames. Some systems use fixed-length frames, whereas other systems provide for variable-length frames (as used herein, the term “frame” refers to both fixed and variable length intervals). Systems using explicit detection transmit frames that include data and a format indication that identifies the transmission format that was used to encode the data.
For example, Universal Mobile Telephone System (UMTS) as defined by the 3rd Generation Partnership Project (3GPP) employs a variable-format transmission scheme using explicit format detection. The set of UMTS specification documents defines a system for broadband, wireless, packet-based transmission of text, digitized voice, video, and multimedia at data rates up to and possibly higher than two megabits per second (Mbps).
Transmission formats in UMTS are denominated transport formats (TF) (these terms are used interchangeably herein). A single UMTS communication link can support different transport channels, or sub-channels, that can be multiplexed both in time and in code. Different services are mapped to different sub-channels. For example, audio may be mapped to sub-channel 1 and video to sub-channel 2. Each sub-channel supports a set of transport formats referred to herein as transport format sets (TFS). A TFS can contain several transport formats. Certain combinations of transport formats are allowed by the service, others are not.
In UMTS, each frame may be encoded using a combination of transport formats, one for each sub-channel, called the transport format combination (TFC). The subset of all permissible combinations that are allowed in any frame is called the transport format combination set (TFCS). A transport format combination indicator (TFCI) is associated with each TFC. The transmitter multiplexes (using time multiplexing) the TFCI within the frame. The receiver extracts the TFCI bits, decodes the TFCI, and then decodes the frame data according to the TFC associated with the decoded TFCI.
Interim Standard 95 (IS-95) is another variable-format transmission scheme. IS-95 is a CDMA-based technology wherein a single service (voice) is efficiently encoded using four different transmission formats. For example, periods of active speech are encoded using a full-rate transmission format, whereas periods of silence are encoded using a ⅛-rate transmission format. Periods such as the beginning or end of a sentence or between words are encoded using a ¼- or ½-rate transmission format.
A received frame may contain errors in the format indication and/or data resulting from noise and interference. Radio communication links are particularly likely to introduce such errors. Robust coding schemes are commonly applied to both the data and the format indication so that errors can be corrected. However, it is always possible that errors will remain in the decoded frame due to particularly bad channel conditions. Corrupted bits in either the format indication or in the data itself will cause an error to occur in the receiver during the decoding process. This is because in the former case, the receiver attempts to decode the frame data using a format different from the format used in the encoding process. In the latter case, the receiver attempts to decode the data using the proper format, but nevertheless fails because the data is corrupted.
The receiver cannot successfully decode data when the data itself is corrupted. In this case, the receiver can only inform the upper application layers that an error occurred so that the data can be retransmitted. However, if the format indication is corrupted, the receiver may still have received uncorrupted data which can be successfully decoded once the correct transport is detected. The receiver therefore must determine which transmission format was actually used to encode the frame data. The problem is further complicated by the fact that the receiver may not know whether a decoding error is due to corruption of the data, corruption of the format indication, or both.
There is therefore a need in the art for an efficient method for detecting the correct transmission format in a variable-format transmission scheme upon encountering a decoding error, so that uncorrupted data may still be successfully decoded.